The long-term goal of this project is to understand the brain mechanisms underlying control of attention. Attention deficit disorders are among the most common developmental disorders in children. Their social and educational consequences incur major costs to the individual and society. Development of an adequate understanding of the underlying neural mechanisms of attention would go a long way toward improving diagnosis and treatment of such disorders. Behavioral and neurophysiological techniques are now available to measure the effects of attention on neuronal activity, and thus provide a way to determine which brain structures are involved in different forms of attentional control. This project tests the basic hypothesis that input to the cerebral cortex from the thalamus plays a critical role in attentional modulation of cortical activity, and that that modulation is critical for behavior. Both physiological (microelectrode recording) and ablation-behavior methods are used in the monkey. The physiological studies examine how changes in attention modulate single neuron activity. Three different behavioral tasks are used to control attention: a task in which relevant stimuli are selected from distractors, a task which directs attention toward different spatial locations, and a task which modulates the general attentive state. Experiment l determines whether varying attention in these tasks modulates discharge of thalamic neurons. Microelectrode recordings are made from pulvinar cells while monkeys perform the behavioral tasks. Experiment 11 tests the hypothesis that thalamic inactivation eliminates attentional modulation of cortical neurons. Microelectrode recordings are made in cortical areas connected with the pulvinar before and after kainic acid lesions, or reversible inactivation (muscimol injection), of the pulvinar. The behavioral studies (Experiment Ill) determine whether pulvinar lesions that eliminate attentional modulation of cortical neurons also impair related forms of attention-dependent behavior. Performance on a visuotopic filtering task, a landmark saccade task, and the Continuous Performance task are compared before and after kainic acid lesions of the pulvinar.